Thread: D2 tyre pressures

Originally Posted by seano87

Will vary dramatically depending on tyres though. Placard settings may be fine with the stock tyres, but put new boots on, even in the same size but different construction (say standard vs LT construction ) it can be a very different story.

I think its trial and error to getting what works best in your own situation. Full stop, end of story.

Sent from my Galaxy Nexus using Tapatalk 2

It might help if you read the thread prior to posting.

In summary:

- Tyres have a design curve with one end point at "Max Load at Max Pressure".
- The curve is essentially linear, so that as loading reduces, the amount of inflation required to support the load reduces proportionally.
- Different tyres have different max. load at max. pressure so will require different pressures for a specific loading.
- Placard pressures are designed to accommodate a specific loading with original fit tyres. In the case of the D2 this is 5 passengers with luggage and a full tank of fuel.

The three methods mentioned in the thread for determining pressures:
- calculated from loading and tyre specs
- determined by foot print length
- calculated from pressure rise after an extended drive

All basically achieve the same thing - matching inflation to load bearing characteristics of the tyre and the load carried.

I'll leave the chicken entrails and superstition to others.


cheers
Paul

A good guideline to hi by its the shape of the the tyres. On a had even surface you want minimal sidewall flex as thus we'll affect your handling and will build unnecessary great in your tyres. So that sexual depend on the tyre, the weight you''re carrying and the wright of your cmd. On sand you want the tyre contact patch to elongate to give you something similar to the action of a track on a tank.
Mud is a different story again. Some will swear that low pressures are the go just like sand, to float across the mud, others will swear on garden pressures to cut into the mud and hurl it away. Really it depends on the tyre and tread pattern....and the driver. Rock crawling you'll need something in between road and sand but probably on the lower side. To mould over the uneven ground, keeping as much rubber in touch with the ground as possible before wrecking the tyres.
Personally i'll run 45-50 psi on the road. My 110 predates power steering. So I need thwarted higher pressures to be able to manouvre it .on a hard dirt road I might lower them to around 40if I am fully loaded, but that will be based on how far I have to go and the weight distribution of the load. On average 14-18 for sand and 16-25for slow rock work. But that's my preferences for my truck and my tyres.

I use the placard pressures as a starting point, then adjust the pressure to suit my driving feel/style, I did this in my old D2 and have done the same in the D4, it really comes down to the individual.

Someone asked the weights over each axle, there's more weight over the rear axle on a D2, I think this is why the front placard pressures are so much lower than the front, if you notice the loaded pressures, only the rear pressure changes.

Baz.

Let's use Cooper Discoverer S/T in LT245/75R16 as an example:

Max Pressure: 80psi/552Kpa
Max. Load: 1380kg per tyre

D2 at maximum front axle loading: 1250kg ( 625kg per wheel) assuming Bullbar, winch, dual batteries, spotties, antennas, etc etc.

Axle load is split across two wheels so each tyre is carrying 625/1380 x 100 = 45.28% of it's maximum load.

Quick and dirty calcs to determine running pressure for this loading: 80psi * 45.28% = 36psi (fr)

NB: I use a spreadsheet that accounts for aspect ratio of the tyre, and this gives an inflation figure of 34psi for the same tyre and loading.

As a second example:

Michelin Latitude Cross 235/70R16 passenger tyres

Max Pressure: 50psi/345Kpa
Maximum Load: 950Kg

Using the same loading of 1250kg on the front axle tells us each tyre is carrying 625/950 x100 = 65.79% of it's maximum load.

Scale the maximum pressure to give inflation for loading: 50psi x 65.79% = 33 psi (fr)

NB: Spreadsheet result accounting for aspect ratio gives 31psi.

So for a 1250kg front axle loading that is fairly close to an ARB bullbar and winch with steel cable, the method I've described in this thread gives a front tyre pressure of 36psi (34 adjusted for aspect ratio) pressure for Cooper Discoverer ST 245/75R16's or 33psi (31psi adjusted) for 235/70R16 Latitude Cross.

Clear as mud?

Originally Posted by Redback

I use the placard pressures as a starting point, then adjust the pressure to suit my driving feel/style, I did this in my old D2 and have done the same in the D4, it really comes down to the individual.

Someone asked the weights over each axle, there's more weight over the rear axle on a D2, I think this is why the front placard pressures are so much lower than the front, if you notice the loaded pressures, only the rear pressure changes.

Baz.

I think LR basically assume you are running stock with the D2. Adding bullbar and winch change the front loading pretty significantly.

Defenders have a far more even weight distribution so the placard pressures have little front/rear differential.

In relation to bagging of side walls part of the spreadsheet I've been using to calculate pressures for my own use includes the degree of deflection in the side walls. With the stock D2 pressures (which represent by my calculations a 1110kg front/1465kg rear weight split) the side wall deflection is identical, so the tyres are bagging an equal amount because both front and rear tyres are supporting the axle load equally. At recommended pressures the sidewalls deflect about 22mm. If you dropped both front and rear to 16psi with this loading the front sidewalls would deflect about 32mm and the rears 50mm. In other-words the rear rims would be around 18mm closer to the road surface than the fronts.

The calculated pressures are a baseline road pressure. Off road, mud and sand, and emergency pressures are based on this. Michelin suggested for the Latitude Cross that these were 20%, 30% and 40% respectively.

Front:
28 - 20% = 22psi Off Road
28 - 30% = 20psi Sand
28 - 40% = 17psi Mud/Emergency

Rear (standard load):
38 - 20% = 30psi Off Road
38 - 30% = 26psi Sand
38 - 40% = 23psi Mud/Emergency

The 20% down for track work is what was recommended when I did the LROCV training so seems fairly well accepted. 30% has worked well for Mallee sand tracks.

These are pretty conservative compared to what I've seen people recommend and run, but I haven't had problems with these pressures.

cheers
Paul

So that sexual depend on the tyre, the weight you''re carrying and the wright of your cmd.

Do you realise how difficult that is to decipher?

Originally Posted by Freightdog

Do you realise how difficult that is to decipher?

Glad I'm not the only one left baffled by the auto spellcheck gibberish.

To answer the question "what were standard weights"...

These were the figures for the Td5 in the 1999 UK specifications brochure:



Kerb EEC weight reflects the vehicle loaded a 80kg driver and full tank of fuel.

As noted above placard pressures are based on 5 occupants and luggage plus full tank.

If you had a standard prefacelift D2 5 seater auto Td5 and didn't carry anything in the rear, you'd be safe running 28psi front/ 32psi rear on stock spec tyres.


cheers
Paul